A characterization of maps in H 1 (B 3 ,S 2 ) which can be approximated by smooth maps
Annales de l'I.H.P. Analyse non linéaire, Volume 7 (1990) no. 4, p. 269-286 
@article{AIHPC_1990__7_4_269_0,
     author = {B\'ethuel, Fabrice},
     title = {A characterization of maps in $H^1 (B^3, S^2)$ which can be approximated by smooth maps},
     journal = {Annales de l'I.H.P. Analyse non lin\'eaire},
     publisher = {Gauthier-Villars},
     volume = {7},
     number = {4},
     year = {1990},
     pages = {269-286},
     zbl = {0708.58004},
     mrnumber = {1067776},
     language = {en},
     url = {http://www.numdam.org/item/AIHPC_1990__7_4_269_0}
}

Bethuel, F. A characterization of maps in $H^1 (B^3, S^2)$ which can be approximated by smooth maps. Annales de l'I.H.P. Analyse non linéaire, Volume 7 (1990) no. 4, pp. 269-286. http://www.numdam.org/item/AIHPC_1990__7_4_269_0/

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